1. Field of the Invention
This invention relates to an improved process and apparatus for simultaneously milling, drying and/or sterilizing a material into a powder or flakes of a desired size, moisture content, and degree of sterilization. This invention also relates to removal of undesired elements or compounds from a material by vaporization or thermal destruction.
2. Prior Art
Generally, four types of dehydrators have been used in industry. The first type of dehydrator is a rotating drum dehydrator which comprises a long tube that is heated and rotated simultaneously. Raw material such as fish scraps from commercial processing of crabs, fish, shrimp, etc. is introduced at one end and is heated and ground while it travels the length of the tube. The material ultimately emerges from the opposite end as a roughly ground powder. This rough powder is then ground to the desired size in a conventional hammer-mill.
A disadvantage of the rotating drum dehydrator is that vital elements such as proteins in the raw material tend to be destroyed. The rotation tube or drum contains pieces of material ranging from large pieces of raw material to small particles of finished dry material. As a result, the raw material is present in a wide range of volume-area ratios and it is difficult to provide optimum drying conditions for all material in the tube or drum. The heat absorption rate of the large pieces is limited by their low area-volume ratio, and the small particles are rapidly dried because of their high area-volume ratio. Thus the heat transfer rate to the large pieces is generally only increased by increasing the temperature of the air or the drum surface, or by increasing the air velocity. Unfortunately, if the air or surface temperatures are elevated, the small particles may be burned or charred. Burning and charring are undesirable and tend to destroy the vital elements sought to be retained in the finished product (e.g., proteins). Similarly, increasing the air velocity generally requires that the air volume also be increased, with attendant loss of efficiency.
Even at optimum conditions with this arrangement, experience has shown that the processing time for most material is several hours. This long processing time generally results in the destruction of heat sensitive protein, vitamins, and nitrates in addition to general burning of the material. In addition, long processing time may often encourage bacteria growth.
The second type of dehydrator grinds the raw material to medium sized pieces. These medium sized pieces are then heated and ground to a pulp, and the pulp is finally mixed with hot air to reduce it to a powder.
This device is a partially successful attempt to overcome the drawbacks of the rotating drum arrangement. By separating the grinding and drying operations, charring is largely eliminated. Nevertheless, the heat transfer rate is generally low because the air velocity relative to the material is low even though the absolute velocity of the heated air is high. The net result is that the machine is generally large, the processing time long, and the heat-sensitive protein may be destroyed because of long exposure to heat. The efficiency of this method also tends to be low.
The third and fourth types of dehydrators operate at high speed and high temperatures and generally grind and dry materials quickly and simultaneously.
In the third type of dehydrator, hot, dry air and the material to be processed are introduced into a mill in a radial direction at one end and the finished powder or meal, vapors, and air are withdrawn through an annular opening around the shaft at the other end of the mill. The mill contains a rotating shaft with blunt hammers or instruments attached. The hot, dry air is usually provided by a refractory furnace. The material is ground and dried in the mill under turbulent conditions. The turbulence, in combination with the action of the exhaust fan at the mill exit, provides the means of separating the finished product from the unfinished material so that the finished product may be withdrawn from the mill.
A disadvantage inherent in the third type of dehydrator is that the magnitude of the aerodynamic forces that may be generated by the hot air on the particles passing through the hot air are limited by practical considerations such as air density and air inlet velocity. The air inlet velocity, in turn, is limited by the pressure gradient that can be generated by an exhaust fan. Limitation of the aerodynamic forces also places a corresponding limitation on the centrifugal forces that may be tolerated in the mill. Consequently, the mill size, speed, and power inputs are limited because the device cannot function if the centrifugal forces are so great that it is impossible for aerodynamic forces to overcome them.
The fourth type of dehydrator is a modified version of the third type but the air enters the mill tangent to, and in the same direction of motion as the rotating hammers or instruments. The raw material enters the mill tangent to, and in a direction that is opposite to the motion the rotating instruments. An example of fourth type of dehydrator is described in U.S. Pat. No. 3,823,877 to Poggie.
The disadvantages of this apparatus are:
A. Some of the raw material entering the mill is generally burned or charred because some of it tends to immediately contact high temperature hot gas upon entering the chamber.
B. Some of the raw material entering the mill is generally forced back into the inlet by the rotating mill instruments.
C. The hot air entering the mill from the furnace is generally limited to about 950.degree. F. (about 510.degree. C.) (because a higher temperature results in an unacceptable level of ash in the finished material) resulting in low volumetric efficiency and low thermodynamic efficiency of the mill and of the furnace.
D. The furnace liner generally requires about one hour to reach a stabilized operating temperature, and as a result:
(1) the machine does not generally reach full capacity for about one hour after starting, and PA1 (2) the moisture content of the finished product exiting the mill does not generally stabilize at the desired moisture percentage for about one hour after the machine starts. PA1 A. Providing an improved process and apparatus for processing organic material while preventing deterioration of the vital elements in the material. PA1 B. Processing any material, including organic material, in a single, high speed operation. PA1 C. Processing material into clean, dry, sterilized powder, meal, pellets, or flakes that are low in ash, high in digestibility, and low in bacteria count. PA1 D. The air inlet to the mill is located such that hot air enters the mill tangent to, and in the same direction as, the motion of the extremities of the rotating mill instruments. Thus the main exhauster needs only to overcome the fan effect of the mill (but not the centrifugal effect of the rotating particles within the mill). PA1 E. Rapidly moving mill instruments do not tend to force the raw material back into the feed mechanism. PA1 F. The incoming hot air does not impinge on, overheat, char, or burn the raw material entering the chamber. PA1 G. The moisture content and particle size of the finished product may be maintained within desired preselected limits. PA1 H. The system operates with increased thermal efficiency, reduced maintenance, and improved temperature control over prior art devices.
E. The high specific heat and low thermal conductivity of the refractory liner causes the response time of the furnace (to variations in raw material temperature, consistency, or moisture content) to be long, thus resulting in over-compensation in the furnace fuel system.
F. The high specific heat and low thermal conductivity of refractory liner in the furnace generally results in a long cool-down period after the machine is shut down. This long cool-down generally causes heavier fuel elements to "coke up" and clog the fuel nozzles and strainers. The clogging of the nozzles generally result in poor temperature regulation, diminished machine capacity, and frequent shut down time to clean the nozzles.
The above-described dehydrators have not proven to be entirely satisfactory for a variety of reasons. In addition to the above described limitations, the first two types usually require that the material be extensively pre-cooked and pressed to remove excess water and oil. This pre-cooking tends to destroy many of the vital elements in the material because of exposure to high heat for a relatively long period of time. In addition, many of dehydrators presently known are large, expensive, and costly to maintain and operate. Furthermore, processing time for the dehydrators presently known is also substantial, and thus the yield of the finished product, per unit time, from these devices is relatively small.